A plant starch is composed of amylase and amylopectin. For example, a usual corn starch is composed of about 20% of amylose and about 80% of amylopectin. Natural amylose contained in the plant starch is a polysaccharide in which many glucoses are mainly bound through α-1,4-glucoside bonds, and it is known that a small amount of branch structures composed of an α-1,6-glucoside bond are also contained. On the other hand, amylopectin is a macromolecule in which a lot of short amylose chains with a degree of polymerization of about 20 are bound through α-1,6-glucoside bonds in a tufted shape. It has been known for a longtime that a linear α-1,4-glucan chain constituting an amylose has a feature of forming a helical structure, and has a function of incorporating various substances into inside of the helical structure (referred to as clathrate function). It has been apparent that blue color developed by the addition of an iodine solution to a starch (iodine-starch reaction) is caused by making a clathrate including iodine atoms inside of an amylose helical structure. There are known, as a substance which can be made to be clathrate by amylose, many inorganic molecules and organic compounds, such as fatty acids and surfactants, in addition to iodine.
Amylose is a polysaccharide having a unique function of clathrate function. However, since it is very difficult to separate amylose from amylopectin in a starch, production of pure amylose is not performed in an industrial scale, and industrial use of amylose is not being advanced. It has recently become possible to enzymatically synthesize pure amylose (Patent Document 1), and a study regarding use of amylose has being proceeding (Patent Document 2). For example, Patent Document 3 discloses molded articles such as fibers and films made of an enzymatically synthesized amylose. These molded articles made of amylase are excellent in biodegradability and biocompatibility since they are easily degraded by amylase in microorganisms or in the body of an animal. Therefore, Patent Document 3 discloses use of molded articles made of amylose in applications which require biodegradability. However, these fibers made only of amylose are not suited for repeated use or applications such as those in which washing is repeated, because of the very high biodegradability. It is described that biodegradability can be controlled by chemical modification. However, chemical modification remarkably suppresses the clathrate function of amylose, these fibers are not suited for use in applications in which the clathrate function of amylose is utilized.
At present, chemical fibers such as polyester fibers are mainly used as fibers. However, since amylose cannot be dissolved in a solvent for materials of conventional chemical fibers, amylose cannot be contained in chemical fibers. Even if being contained is possible, since chemical fibers are not compatible with amylose, they cannot be mixed in a molecular level, and thus causing phase separation.
On the other hand, cellulose is a polysaccharide constituting plant cell walls and in which many glucoses are linked through β-1,4-glucoside bonds. Cellulose is a polysaccharide which is by far excellent in stability as compared with a starch, and it is a main raw material for, for example, clothing, nonwoven fabric, paper and the like.
If it is possible to impart the clathrate function of amylose to cellulose, there is a possibility that a novel functional material having features of both substances can be developed. Patent Document 4 discloses a method in which cellulose fibers are coated with amylose by applying an aqueous amylose solution to a nonwoven fabric made of cellulose fibers. This method is easy to operate and is practical. However, this method has a problem that the product also cannot endure repeated use since amylose attached on the cellulose surface is easily lost by an operation such as washing.
One of cellulose fibers includes rayon. Rayon is fiber which is formed while regenerating cellulose from a solution (viscose) which is prepared by dissolving cellulose using carbon disulfide. Due to the feature in their production, rayon has such a feature that various functional substances can be contained in rayon by adding these functional substances to viscose, and thus making it possible to impart functions to the rayon. There have hitherto been disclosed rayon fibers containing chitosan (Patent Document 5), rayon fibers containing complex metal oxide microparticles (Patent Document 6), rayon fibers containing bincho-tan charcoal microparticles (Patent Document 7), rayon fibers containing an anionic macromolecule (Patent Document 8), and the like. However, when rayon is produced by adding a functional substance to viscose, the functional substance is coated with the rayon fibers, and thus the expected functionality can not be sufficiently used in some cases. In contrast, the shape of a functional component has been devised so as to expose the functional component on the surface of rayon fibers (Patent Document 7). However, this method is not necessarily applicable to any functional component. There has been another devise such as reduction processing treatment of exposing a functional component by decomposing cellulose on the surface of rayon fibers using an enzyme (Patent Document 6). However, this method also has problems such as deterioration of texture of the rayon fibers and a decrease in mechanical strength of the rayon fiber. There is also a problem such as deterioration of washing resistance caused by exposure of the functional component.
Patent Document 8 discloses that, when a macromolecular substance is contained in rayon, the molecular weight of the macromolecular substance is suitably from 10,000 to 500,000 from the viewpoint of yield of remaining in a rayon. However, the technique described in Patent Document 8 merely uses the macromolecular substance so as to retain ionic functional groups in rayon, and the structure of the macromolecular substance in the rayon is not an issue. When a macromolecular substance such as amylose has a function, in addition to that the function is not exerted because of the macromolecular substance is coated with rayon as described above, due to the structural change of the macromolecular substance and the like, the function may not be sufficiently exerted. It is very difficult to predict the results since the structural change of the macromolecular substance varies depending on the kind of and the production method of the macromolecular substances. It is known that the structure of the macromolecular substance, rather, changes easily by physiochemical stimulation, and thus it is considered that the function of the macromolecular substance is lost as a result of the structural change caused by being contained in rayon. Furthermore, Patent Document 8 aims at preventing the antimicrobial effect from reducing by loss of a quaternary ammonium salt compound during repeated washing, and therefore the object of Patent Document 8 is quite different from that of the present invention. An antimicrobial agent binds through an ionic bond in the method described in Patent Document 8, whereas, in the present invention, an antimicrobial agent is made to be clathrate in amylose. Therefore, these techniques of retaining the antimicrobial agent are completely different. Furthermore, ionic bonds can bond the antimicrobial agent only to a portion where an ion is present, whereas, in the present invention, the antimicrobial agent can be made clathrate in various portions of the amylose chain. Therefore, the antimicrobial agent can be bound in an amount larger than that in a conventional manner if the content of the substance to be bound to a quaternary ammonium salt in the fiber is the same as the content of amylose.
Cyclodextrin is known as a compound having ability to form a clathrate. However, since cyclodextrin has a low molecular weight and is dissolved in water, in the case of viscose rayon made by a wet spinning method, cyclodextrin is eluted in a spinning bath during forming into fibers, and thus the yield of retaining in the fibers is not high. Even if a small amount of cyclodextrin can be contained in the rayon fibers, the cyclodextrin is eluted easily from the fibers, and thus the obtained fibers are inferior in stability.
Therefore, there is disclosed a method in which cyclodextrin is bound to the fiber surface through a chemical bond (Non-Patent Document 1). However, this method can impart the ability to form a clathrate only to the fiber surface. There is also a problem that when the fibers are, for example, exposed to an acid or alkaline solution, the chemical bond is cleaved to elute cyclodextrin on the fiber surface and the ability to form a clathrate is easily lost.
There is disclosed, as a method of allowing fibers to contain cyclodextrin, a method in which a conjugate obtained by bonding cyclodextrin to the end of a polyester-based polymer through a chemical bond is mixed with a thermoplastic resin to obtain fibers (Patent Document 10). However, cyclodextrin is bound only to the end of the polymer and thus the amount of bound cyclodextrin is small, and cyclodextrin coated with the polymer cannot exhibit the ability to form a clathrate. Therefore, there is a problem that the clathrate amount of a guest substance is limited. If it is tried to increase the amount of bound cyclodextrin by this method, the degree of polymerization of the polymer has to be lowered, and, as a result, causing a problem that the fiber strength is insufficient. In contrast, there is also disclosed a method in which a plural of cyclodextrins are chemically bound to a polymer molecule (Patent Document 11). However, this method requires complicated steps of synthesizing a cyclodextrin derivative and then polymerizing the cyclodextrin derivative, leading to much labor in synthesis and poor efficiency. Furthermore, it is unclear whether or not the obtained polymer can be formed into fibers and the obtained fibers have sufficient ability to form a clathrate after molding.
As described above, it becomes necessary that a polymer substance having ability to form a clathrate, such as amylose is contained in the entire fibers so as to impart the ability to form a clathrate to the entire fibers. However, there are many problems to be solved, including a loss of the ability to form a clathrate of amylose caused by being contained into the fibers, and a change in physical properties of the fibers caused by containing of amylose.
When amylose is used as a functional component, natural amylose cannot be completely dissolved under an alkaline condition at the time of the production of rayon, even if it is tried to add the natural amylose into rayon in the same method as in Patent Documents 5 to 8. As a result, an amylose-containing rayon can not be produced because of the occurrence of nozzle clogging or the like at the time of the production of rayon.
On the other hand, with respect to a method of producing iodine, there are known, as a method of obtaining iodine from brine or the like, a blowing-out method in which iodide ions, an iodine compound and the like are converted into iodine molecules by chemical reaction or the like, and the iodine molecules are vaporized out into the air and then recovered by absorbing with an absorption liquid; an activated carbon adsorption method in which iodine is recovered by adsorbing with activated carbon; copper and silver methods in which iodine is reacted with copper or silver and then recovered as a precipitate of copper iodide or silver iodide; an ion exchange resin adsorption method in which iodide ions are converted into iodine molecules or polyiodide ions, and then the iodine molecules or polyiodide ions are recovered by adsorbing with an ion exchange resin; and the like.
Among these methods, a blowing-out method and an ion exchange resin adsorption method are mainly used. However, there is a problem that the collection ratio of iodine is from 80 to 90% and iodine remains in the brine although in a low concentration after iodine collection, together with dissolved matters such as ammonia and bromine. In the ion exchange resin adsorption method, there is a method that a large amount of an alkali is required when detaching iodide ions from the resin, and thus causing deterioration of the resin. Furthermore, any of these conventionally known methods have a problem that complicated steps are required. Therefore, it has been desired to provide a method of adsorbing and recovering iodine, which consists of simple steps and can adsorb almost 100% of iodine from brine, and can also recover and industrially use the adsorbed iodine.
There have hitherto been disclosed, as a substance capable of stably retaining iodine, for example, an amylose powder (Patent Document 4) and a CD polymer (Patent Document 9).
It is known that iodine is made clathrate in amylose (for example, Patent Document 4). However, it is not possible to use those, in which iodine is made clathrate in an amylose powder, for the purpose of recovering iodine from brine since iodine is released from them in an aqueous solution.
Furthermore, in order to retaining iodine by stably making clathrate in an amylose powder, a metal halide is required. The metal halide is easily lost from the amylose powder. Therefore, when the amylose powder retaining iodine is used in a molded article, there may arise a problem of lowering of retaining stability of iodine due to lose of metal halide.
Patent Document 9 discloses a method in which a CD polymer is used as an iodine adsorption material. However, the CD polymer has problems such as low adsorption capacity and higher cost compared to an ion exchange resin. Furthermore, the CD polymer can retain iodine only in the form of triiodide ions (I3−). The properties of iodine, such as antimicrobial and oxidation potencies, are exerted in the form of iodine molecules (I2). Therefore, in the CD polymer, the proportion of the iodine molecules (I2) of the entire iodine is low, such as ⅔, and thus there is a problem of low retaining amount of effective iodine (I2) and a problem of poor retaining stability. It is expected that substances retaining iodine molecules are used under a high humidity environment, such as disposable masks, for the purpose of exerting their antimicrobial property and oxidation potency. It is noted that masks are referred to as face masks or hospital masks in English. However, the conventional CD clathrate substances are said to release iodine molecules by humidification, and thus safety for humans arising from inhaling the iodine molecules is also an issue to be concerned.
As described above, there have been required to provide molded articles which can retain iodine in the state of iodine molecules or polyiodide ions without adding metal halide. There have also been required to provide molded articles which can retain iodine stably even under a high humidity environment.